Many commercial jet aircraft are subject to governmental regulations that limit the permissible noise levels generated by the aircraft near airports. One source of noise from jet aircraft is engine noise that propagates forward from the engine through the air intake or inlet. One method for attenuating inlet noise is to line the inlet with an acoustic liner that includes a honeycomb core sandwiched between a perforated front sheet and a solid back sheet. Accordingly, each cell of the honeycomb core has an opening at the front sheet that defines a Helmholtz resonator. The perforated front sheet is aligned with the inlet flow so that the sound waves in the inlet pass through the front sheet and into the honeycomb core where they are dissipated. The acoustic liner typically extends along the inner surface of the inlet to the engine.
One problem associated with the acoustic core is that it can collect water and water vapor through the perforated front sheet. Water in the acoustic core can freeze and expand (e.g., when the core cools down as the aircraft gains altitude), causing the acoustic core to delaminate or otherwise undergo structural failure. One approach for addressing this problem is to provide the acoustic core with a series of internal channels that collect the water and drain the water through one or more drain holes. However, the drain holes themselves can create additional problems. For example, during some flight (and/or ground operation) conditions, the pressure at the perforated front sheet is less than the pressure at the drain holes. As a result, air tends to get sucked into the acoustic core through the drain holes. From the acoustic core, this air transpires out through the perforated front sheet, where it can disrupt the main air flow through the inlet, particularly at high inlet angles of attack and/or high cross-wind conditions and/or high engine power settings. The disruption in inlet air flow can in turn reduce engine performance, resulting in inefficient aircraft operation.
The foregoing transpiration air flow problem is exacerbated as the amount of acoustic treatment provided in the inlet increases in response to environmental and regulatory pressures to further reduce aircraft inlet noise. For example, the acoustic treatment has typically been placed close to the engine fan, but, in an effort to further reduce inlet noise, recent installations include extending the acoustic treatment forward from the fan up to and forward of the inlet throat. The air pressure at the inlet throat is typically lower than anywhere else in the inlet, which can further increase the tendency for transpiration flow to enter the inlet through the drain holes and the acoustic core.
To address the increased tendency for transpiration flow to enter the inlet, one approach has been to cover the perforations in the face sheet near the locations of the drain holes. This has the effect of reducing the amount of transpiration flow that can pass directly from the drain holes through the acoustic core and into the inlet flow field via the perforated face sheet. However, this approach suffers from several drawbacks. One such drawback is that blocking the perforated face sheet in selected regions reduces the overall effectiveness of the acoustic treatment, and therefore reduces the noise attenuation provided by the acoustic core. Another drawback is that blocking selected portions of the perforated face sheet adds to the complexity of manufacturing the inlet because the acoustic treatment is no longer uniform. For example, special care must be taken to align unperforated sections of the face sheet with the drain holes, and ensure that perforated sections of the face sheet are not aligned with the drain holes. As a result, the cost of manufacturing the inlet can be undesirably increased.